This invention relates to a radio receiver which is selectively called as a selectively called radio receiver to receive a message signal with battery savings. Such a radio receiver is typically used either as a pager receiver of a radio telephone network or in a mobile station of a mobile communication network.
By the radio receiver which is of the type described and is energized by a battery, a radio signal is received as a received signal with a battery saving operation of fully using the battery only while necessary. The radio signal includes a selective call signal which is specific to the radio receiver under consideration and is used in selectively calling the radio receiver in question.
A basic format for the radio signal is specified in a reference which was published Autumn 1980 under the title of "Standard Message Formats for Digital Radio Paging" by the Post Office Code Standardisation Advisory Group (POCSAG) of London, the United Kingdom. In accordance with the format of the POCSAG reference which format will later be exemplified more in detail, the radio receiver is put in operation in a time division fashion. The radio signal and consequently the received signal derived from the radio signal with no errors will herein be referred to alternatively as a POCSAG signal.
The POCSAG signal has a specific bit rate at a time for at least specific receivers including the selectively called radio receiver among the radio receivers of the network and is given by a plurality of serial batch streams, each comprising a preamble signal and a succession of batches following the preamble signal. The batches comprise synchronization signals, respectively. In each batch, a sequence of "groups" of bits follows the synchronization signal of the batch under consideration. As a consequence, one of the synchronization signals of the batches immediately follows the preamble signal in each batch stream as an immediately following signal of such synchronization signals.
As a specific group, one of the groups of the batch streams carries an individual call signal indicative of a particular receiver of the specific receivers. In order to be received by the particular receiver, an individual message signal is carried by the groups following the specific group in the batch stream or streams and including the specific group or groups other than the specific group by which the individual call signal is carried. The selective call signal is carried as such an individual call signal by a predetermined group of the batch streams. As such an individual message signal, the first-mentioned message signal is herein referred to alternatively as a particular message signal and is carried by the groups which follow the predetermined group as described above.
In the manner which will later be described in greater detail, the radio receiver includes a receiving unit supplied with the received signal to produce a demodulated signal of the specific bit rate with the battery saving operation. A synchronizing unit establishes and keeps synchronism between the demodulated signal and operation of the receiving unit. More particularly, the synchronizing unit detects the preamble signal and the immediately following signal in each batch stream to intermittently release or suspend the battery saving operation and to thereby render the radio receiver alive, namely, to put the receiving unit in full operation of detecting the selective call signal and, after the selective call signal is received, in full operation of receiving, together with the synchronization signal or signals, the particular message signal.
It should be noted in connection with the foregoing that the preamble signal is included only once in each batch stream. The received signal may be subjected to fading to give rise to a failure in detection of the preamble signal. Alternatively, the received signal may be adversely affected by noise to include a false signal which is misdetected as the synchronization signal while the selective call signal and the particular message signal are received. Such a failure and/or a misdetection result in collapse of the synchronism. Once the synchronism is lost, the receiving unit can no more detect the selective call signal and/or the particular message signal.
The radio receiver therefore comprises a bit rate detecting unit for detecting the specific bit rate to produce a bit rate detection signal for use in assisting or supplementing operation of the synchronizing unit. More specifically, the bit rate detecting unit detects the specific bit rate to restors the synchronism.
Bit rate detection may, however, give rise to occurrence of false synchronism, namely, to an increase in undesired occurrences of misdetection of the bit rate if the bit rate is detected with a high sensitivity in order to avoid failure of detection of the bit rate. The misdetection may take place because the received signal may have a pseudo or false bit rate due to the adverse effects. The misdetection results in an objectionable amount of consumption of the battery.
It is possible to preliminarily select for each radio receiver of the network a parameter for determining the sensitivity of bit detection to reduce the undesired occurence of misdetection. The sensitivity, however, varies to a great extend in dependency on a manner in which the radio signal is transmitted from a base station of the network. As a result, it is impossible to change the parameter unless the radio receivers of the network are all withdrawn for adjustment of the parameter. Consequently, use of the parameter is objectionable.